For improvement in fuel efficiency and reduction in weight of automobiles, it has been increasingly demanded to improve dent resistance of an outer part and to reduce the thickness thereof by use of a high strength steel sheet for an automobile body.
As used for the outer part of the automobile body, a cold-rolled steel sheet is required to have good properties in terms of tensile strength, yield strength, press formability, spot weldability, fatigue resistance, corrosion resistance, etc.
In particular, the corrosion resistance has been recently required for extension in lifetime of components for the automobile.
Steel sheets for improvement in corrosion resistance can be generally classified into two types, i.e. a electroplated steel sheet and a galvannealed steel sheet.
In comparison with the steel sheet for galvannealing, although the steel sheet for electroplating has better plating properties and superior corrosion resistance, it is rarely used due to its very high price. Therefore, the galvannealed steel sheet is generally used in the art, and required to have improved corrosion resistance.
In recent years, most steelmaker in the world have produced the galvannealed steel sheets as materials for the automobiles, and supplied them to automobile manufacturers. Accordingly, new techniques capable of securing superior corrosion resistance above a conventional level have been continuously developed and increasingly used.
Generally, the steel sheet exhibits incompatible characteristics in terms of strength and formability. The steel sheets capable of satisfying both characteristics include multi-phase structure based cold-rolled steel sheets and bake hardenable cold-rolled steel sheets.
In general, the multi-phase structure based cold-rolled steel can be easily manufactured, and has high tensile strength at the level of 390 MPa or more. Furthermore, despite the higher tensile strength as compared with general materials for the automobiles, the multi-phase structure cold-rolled steel has high elongation, which is a factor of stretchability. However, it has a low average r-value as a factor of press formability of the automobiles, and comprises excessive amounts of expensive alloying elements such as Mn, Cr and the like, causing an increase in manufacturing costs.
The bake hardenable cold-rolled steel has yield strength approaching that of mild steel upon press forming of the steel which has a tensile strength of 390 MPa or less. Thus, it has superior ductility, and spontaneously increases in yield strength upon paint baking after press forming. With these merits, the bake hardenable cold-rolled steel is spotlighted as ideal steel overcoming a disadvantage of conventional steel, of which formability is deteriorated in proportion to an increase of strength.
Bake hardening is a process which employs a kind of strain aging phenomenon occurring as interstitial elements, such as solute nitrogen or solute carbon, dissolved in a solid solution state in the steel pin dislocations created during deformation. When the steel has high amounts of solute carbon and nitrogen, a bake hardenability advantageously increases, but a natural aging property also increases due to such high amount of dissolved elements, thereby deteriorating the formability. Thus, it is very important to optimize the amount of dissolved elements in the steel.
As a method for manufacturing the bake hardenable cold-rolled steel sheet, batch annealing and continuous annealing are generally used.
Generally, the bake hardenable cold-rolled steel sheet is produced by batch annealing with a low carbon, P-added, Al-killed steel through coiling of a hot-rolled steel sheet at low temperatures. Specifically, when manufacturing the bake hardenable cold-rolled steel sheet using the Al-killed steel, the hot-rolled steel sheet is coiled at a low temperature in the range of 400˜500° C., followed by batch annealing the hot-rolled steel to have bake hardenability (BH) value of about 40 to 50 MPa. This is because the batch annealing enables both formability and bake-hardenability to be obtained more easily at the same time.
Meanwhile, for the continuous annealing, since the P-added Al-killed steel is cooled at a relatively rapid rate, it is easy to secure the bake-hardenability, but there is a problem in that the formability is deteriorated due to rapid heating and a short annealing process. Thus, the continuous annealing-based steel sheet is restricted in use for the outer part of the automobile body, which do not require workability.
Recently, with rapid advances in steel manufacturing technique, it becomes possible to optimize the amount of dissolved elements in the steel, and to manufacture bake hardenable cold-rolled steel sheets with superior formability through addition of various carbide and nitride formation elements, such as Ti or Nb, to the Al-killed steel, thereby satisfying increasing demands for the bake hardenable cold-rolled steel sheets, which can be used for the outer part of the automobiles requiring dent resistance.
Japanese Patent Publication No. (Sho) 61-0026757 discloses an ultra low carbon cold-rolled steel sheet, which comprises: 0.0005˜0.015% of C; 0.05% or less of S+N; and Ti and Nb or a compound thereof. Japanese Patent Publication No. (Sho) 57-0089437 discloses a method for manufacturing a bake hardenable cold-rolled steel sheet, which uses Ti-added steel comprising 0.010% or less of C, and has BH value of about 40 MPa or more.
The methods of the disclosures are to impart the bake hardenability to the steel sheet while preventing deterioration in other properties of the steel sheet by appropriately controlling the amount of dissolved elements in the steel through control of the added amount of Ti and Nb or the cooling rate during annealing.
However, for the Ti-added steel or Ti and Nb-added steel, it is necessary to strictly control the amounts of Ti, N and S during manufacture of the steel to ensure an appropriate BH value, causing an increase of manufacturing costs.
Furthermore, the Nb-added steel described above has problems in that operability is degraded due to high temperature annealing, and in that manufacturing costs are increased due to addition of specific elements.
On the other hand, U.S. Pat. Nos. 5,556,485 and 5,656,102 (Bethlehem Steel, Co., USA) disclose methods of manufacturing a bake hardenable cold-rolled steel sheet from Ti—V based ultra low carbon steel, which comprises 0.0005˜0.1% of C; 0˜2.5% of Mn; 0˜0.5% of Al; 0˜0.04% of N; 0˜0.5% of Ti; and 0.005˜0.6% of V.
Generally, since V is more stable than the carbide and nitride formation elements such as Ti and Nb, it can lower an annealing temperature. Hence, carbide, such as VC and the like, created during high temperature annealing can impart the bake hardenability to the steel via re-melting even with the lower annealing temperature than that for the Nb-based steel.
However, although V can create the carbide such as VC, since it does not sufficiently improve the formability due to its significantly low re-melting temperature, Ti is added in an amount of about 0.02% or more for the purpose of enhancing the formability, as disclosed in the publications. Thus, the methods disclosed in the publications are disadvantage in terms of aging resistance due to coarse crystal grains, and suffer from an increase in manufacturing costs due to addition of large amounts of Ti.
Meanwhile, various methods of manufacturing the bake hardenable cold-rolled steel sheet through addition of alloying elements are disclosed in Japanese Patent Laid-open Nos. (Hei) 5-0093502, (Hei) 9-0249936, (Hei) 8-0049038 and (Hei) 7-0278654.
Japanese Patent Laid-open No. (Hei) 5-0093502 discloses a method for enhancing the bake hardenability through addition of Sn, and Japanese Patent Laid-open No. (Hei) 9-0249936 discloses a method for enhancing the ductility of steel by relieving stress concentration on grain boundaries through addition of V and Nb in combination.
Japanese Patent Laid-open No. (Hei) 8-0049038 discloses a method for enhancing the formability through addition of Zr, and Japanese Patent Laid-open No. (Hei) 7-0278654 discloses a method for enhancing the formability by increasing the strength while minimizing deterioration of work hardening index (N-value) through addition of Cr.
However, these methods only give attention to improvement in the bake hardenability or the formability, and do not disclose the problem of deterioration in aging resistance resulting from the improvement in bake hardenability, and the problem of secondary work embrittlement resulting from increase in content of P, which is necessarily added to increase the strength of the bake hardenable steel.
Generally, the increase of bake hardenability causes the deterioration of aging resistance at room temperature. In particular, the inventors have found that, with an increase in content of P added for high strength of the steel, the steel is degraded so much more secondary work embrittlement resistance even in the case of the bake hardenable steel which comprises dissolved carbon in the steel.
For example, when P was added in an amount of 0.07% to produce bake hardenable steel of the tensile strength at the level of 340 MPa, a ductility-brittleness transition temperature (DBTT) of the steel as a reference to determine the secondary work embrittlement was −20° C. at a drawing ratio of 1.9. When P was added in an amount of about 0.09% to produce high strength steel at the level of 390 MPa, the DBTT of the steel was in the range of 0˜10° C., from which it can be concluded that the steel is significantly deteriorated in secondary work embrittlement resistance.
In the methods described above, although boron (B) is added in an amount of about 5 ppm and expected to improve the secondary work embrittlement resistance, the excessive P content limits improvement in DBTT through addition of B.
Furthermore, if B is excessively added to the steel to improve the secondary work embrittlement resistance, the properties of the steel are deteriorated due to the excessive content of B. Thus, there is a limit in the amount of B which can be added to the steel.
Since the steel must have a DBTT of −20° C. or more to prevent the secondary work embrittlement, there are needs to investigate new compositions other than B for the bake hardenable steel and new manufacturing conditions therefor.